Numerous systems are in use for making foam for various applications where the physical properties of foam (low density and high contact area, thixotropical qualities) provide a significant improvement over the intrinsic qualities of the substance dispensed in liquid form. By way of example, systems for producing foam are used for applying an active substance on a surface that is to be cleaned, degreased, asepticized, depolluted, chemically deactivated, or neutralized.
In all applications that use a foam, it is always desirable to reduce bubble size pro rata the gas in the liquid. This reduction in bubble size increases the contact area with the medium that is to be treated per unit mass of active substance. Numerous foam dispersion systems use a nozzle which atomizes the substance at the outlet from the apparatus and gives rise to a foaming effect by spraying at high speed a large number of fine droplets containing a substance that foams on impact.
Over the last few years, foam generator systems have progressed with the introduction of systems that make it possible, in particular, to inject a gas and a liquid simultaneously into a liquid-gas-liquid dispersion space which can be adjustable to adjust the fraction of added gas, as described in WO 95/31287. However, even if the gas content can thus be significant, there is no action of bubbles being split up by cavitation and so bubble size remains visible to the naked eye.
Others, e.g. U.S. Pat. No. 5,085,371, make use of mechanical elements in the form of obstacles (a grid in the document mentioned) or guides that establish turbulent conditions instead of laminar conditions, thereby enhancing gas-liquid mixing.
The efficiency of such a system can be verified completely and easily by determining the percentage of active substance that is needed in solution to accomplish a given action which can be quantified per unit area.
Although the above-mentioned solutions significantly improve the production of foam compared with more primitive systems, they do not achieve optimum mixing and fineness of gas bubbles in the liquid.
Furthermore, when using an atomizing nozzle, the foam is always formed after it has left the system, which causes bubbles to be formed under static atmospheric pressure and as a result bubble size is large and the contact area and the wetting activity of the foam cannot be optimized. In most washing systems in use, the outlet nozzle is adapted to atomize the fluid by increasing the pressure and speed parameters of the fluid, thereby reducing static pressure, since the potential energy of static pressure is transformed in this way into kinetic energy.
When a liquid-gas fluid passes through a diverging portion, its speed decreases and its static pressure conditions exceed a certain value, so gas bubbles can no longer continue to expand, so under the effect of pressure they then implode and break up into a plurality of cavities of much smaller dimensions. This implosion is accompanied by shock waves that are very large compared with the dimensions of the cavities associated with a high speed of the walls of said cavities. This phenomenon is studied in detail by Hammit, "Cavitation and multiphase phenomena", Mac-Graw Hill 1980. In particular, he describes cavitation phenomena in a conical diverging portion of a Venturi tube. However, in most existing uses, the cavitation phenomena must be avoided since otherwise proper operation of the nozzles is hindered.